Many currently existing sources of hydrocarbon fuels such as natural gas, crude oil, and coal for liquidation contain significant amounts of sulfur. Due to environmental and other concerns, the sulfur present in many of these hydrocarbon fuels must be removed before the fuels can be utilized. For example, in refinery operations, sulfur is typically extracted from crude oil in the form of hydrogen sulfide mixed with carbon dioxide, ammonia and other gases.
Extraction of hydrogen sulfide from a sour gas stream may be accomplished by means of any suitable adsorbent or absorbent. Preferably, where the sour gas feed stream contains a high fraction of non-reactive gases, an absorption solvent that is selective for hydrogen sulfide in the presence of the non-reactive gas fraction is used. More specifically, extraction of hydrogen sulfide from the sour gas stream includes the use of an absorption solvent which is highly selectively for hydrogen sulfide in the presence of carbon dioxide which tends to readily absorb along with hydrogen sulfide. Suitable absorption solvents include aqueous solutions of organic or inorganic alkaline compounds which selectively absorb the hydrogen sulfide in a heat reversible relationship. Organic hydrogen sulfide absorbents which may be employed in formulating the solvent solution include organic amines. Such compounds may take the form of substituted or unsubstituted aliphatic, cycloalkyl, aryl or heteraryl amines. Certain basic amino acids and amides will absorb hydrogen sulfide and may also be employed. Suitable organic compounds thus include dimethyl formamide, morpholine, and various amino alcohols and particularly the alkanolamines, including the mono- or poly-alkanolamines. Thus, examples of suitable absorbents include aqueous solutions of monoethanolamine, diethanolamine, triethanolamine, alkylalkanolamines such as methyldiethanolamine and ethylaminoethanol, diisopropanolamine, di-n-propanolamine, n-propanolamine, isopropanolamine, cyclohexylaminoethanol, and 2-amino-2-methyl-1-propanol. The tertiary amines exhibit a greater tendency for preferential absorption of hydrogen sulfide in the presence of carbon dioxide than do the primary amines and to a lesser extent the secondary amines. Suitable tertiary amines include methyldiethanolamine, triethanolamine and methyldiethanolamine. Preferably, the alkaline absorbent for hydrogen sulfide is present in the aqueous solution in an amount within the range of 20-60 percent. Such amines may be employed in aqueous solution with an organic or inorganic acid. For example, the absorbing solvent may take the form of tertiary amines such as methyldiethanolamine or triethanolamine in an aqueous solution containing a minor amount of an acid such as phosphoric acid.
Hydrogen sulfide and other components extracted from the sour gas stream with an absorbent are stripped from the absorbent by direct or indirect heating in a suitable vessel (regenerator) such as a packed or tray type column to produce an acid gas containing gaseous hydrogen sulfide which is then conveyed to a sulfur recovery unit ("SRU"). While the use of an absorbent solvent which is selective for hydrogen sulfide in the presence of the other components of the sour gas stream tends to increase the concentration of hydrogen sulfide in the acid gas, in many instances the acid gas will also contain significant amounts of carbon dioxide. In some cases, the concentration of carbon dioxide in the acid gas will be several time greater than the concentration of hydrogen sulfide. The acid gas stream containing hydrogen sulfide is subsequently routed to one or more sulfur recovery units and the absorbent is recycled to the absorber unit.
In the sulfur recovery unit the hydrogen sulfide is converted to elemental sulfur via the Claus process, which takes advantage of the reactivity of sulfur dioxide and hydrogen sulfide to produce elemental sulfur by bringing these two agents together in approximately stoichiometric proportions of two parts hydrogen sulfide to one part sulfur dioxide. Recovery of elemental sulfur from sulfur containing gas streams by the Claus process is a widely practiced procedure wherein elemental sulfur is produced by the well known Claus reaction as follows: EQU 2H.sub.2 S+SO.sub.2 &lt;- - - &gt;2H.sub.2 O+3S
Under normal circumstances, the feed gas (acid gas) to the Claus process contains a substantial portion of hydrogen sulfide which is partially oxidized by combustion to produce sulfur dioxide in an amount approximately satisfying the stoichiometric relationship indicated above. Sufficient oxygen is supplied to the hydrogen sulfide containing stream in a combustion zone to oxidize about 1/3 of the hydrogen sulfide to sulfur dioxide via the following reaction: 2H.sub.2 S+30.sub.2 - - - &gt;2 H.sub.2 O+2SO.sub.2. The hydrogen sulfide and sulfur dioxide react to generate water and elemental sulfur. Additionally, hydrocarbons contained in the acid gas stream are converted to organic sulfur compounds such as carbon disulfide or others, depending upon the hydrocarbon concentration.
Alternatively, approximately 1/3 or more of the acid gas stream is diverted to a combustion zone where oxidation of the hydrogen sulfide is carried out in the presence of an amount of oxygen adequate to provide the required amount of sulfur dioxide. The remaining portion of the acid gas stream is not treated so that when the split portions of the gas stream are recombined, the combined gas stream contains hydrogen sulfide and sulfur dioxide in the approximately stoichiometric ratio.
As indicated above, three moles of oxygen are required to combust two moles of hydrogen sulfide to water and sulfur dioxide. The sulfur dioxide then reacts with the remaining hydrogen sulfide in one or more reactor(s) or reaction zone(s) over a catalyst such as bauxite, alumina or titanium dioxide at elevated temperatures to produce elemental sulfur and water vapor. The organic sulfur compounds are also oxidized in the reactor to hydrogen sulfide and carbon dioxide. Additionally, during combustion of the hydrogen sulfide, a portion of the hydrogen sulfide in the feed gas dissociates to free hydrogen and elemental sulfur, H.sub.2 S+heat - - - &gt;H.sub.2 +S via thermal decomposition. The residual hydrocarbons present in the gas stream are oxidized to form carbon monoxide and water vapor.
Oxygen is normally supplied to a Claus unit as pressurized combustion air which typically contains about 21 mole % oxygen and 79 mole % nitrogen on a dry basis. Thus, for every mole of oxygen used to combust hydrogen sulfide, approximately 4 moles of nitrogen are introduced into the gas stream. Although nitrogen is inert under typical Claus reaction conditions, the nitrogen contained in the combustion air represents a significant portion of the hydraulic loading of a Claus type sulfur recovery unit.
Additionally, the acid gas stream may also contain other gases, such as carbon dioxide which are inert under typical Claus reaction conditions. Some sour gases contain substantial amounts of carbon dioxide which is readily absorbed in the medium used to scrub hydrogen sulfide from the sour gas. Carbon dioxide absorbed by the absorbent or scrubbing medium is thermally separated, along with hydrogen sulfide, when the absorbent is regenerated and contributes to volume of acid gas generated and to the hydraulic loading on the sulfur recovery unit. Depending upon the relative concentrations of carbon dioxide and hydrogen sulfide in the sour gas, the acid gas generated by stripping the absorbent may contain several times the amount of carbon dioxide as hydrogen sulfide. Consequently, carbon dioxide in the acid gas stream can have a significant impact on the hydraulic loading of the sulfur recovery unit. The thermodynamics of the Claus reaction render the recovery of elemental sulfur very sensitive to the concentration of hydrogen sulfide in the acid gas stream. The sulfur recovery increases directly with the hydrogen sulfide concentration. Higher hydrogen sulfide concentration in the acid gas stream also promotes greater flame stability of the burner in a sulfur recovery unit.
The amount of oxygen supplied to a Claus type sulfur recovery unit must be controlled to compensate for variability in the volume and composition of the acid gas feed stream(s) to the unit in order to maintain the desired stoichiometric ratio of two moles of hydrogen sulfide per mole of sulfur dioxide in the gas stream entering the reaction zone. The total gas flow through a sulfur recovery unit is, however, limited by the hydraulic capacity of the system. Thus, carbon dioxide contained in the acid gas stream along with nitrogen contained in the combustion air tend to limit the effective capacity of a sulfur recovery unit. Moreover, from a design standpoint it is not necessarily desirable to design a sulfur recovery unit capable of combusting, with air, the peak volume of acid gas that must be processed due to increased equipment cost and capital expenditure. Additionally, in the case of existing plants, the hydraulic limitations of a sulfur recovery unit may be exceeded if acid gas production is increased due to changes in the process or feedstock. Thus, there exists a need for a method and apparatus for controlling the hydraulic loading on a Claus type sulfur recovery unit.
Taggart et al., U.S. Pat. No. 4,919,912, discloses a process for treating sulfur containing gas streams using the Claus reaction in which a recycled stream containing a reactive component is employed in a negative feedback mode to maintain the sulfur producing Claus reaction at approximately equilibrium conditions. The feedstream may contain hydrogen sulfide or sulfur dioxide in a minor amount in an inert gas background. The feedstream to the reaction zone contains a stoichiometrically excess amount of sulfur dioxide for the Claus reaction. Effluent from the reaction zone is passed to a hydrogenation zone where the sulfur dioxide is converted to hydrogen sulfide. Hydrogen sulfide is extracted from the hydrogenation zone effluent and recycled to the Claus reaction zone.
Bond et al., U.S. Pat. No. 3,963,443, discloses a gas mixer and reactor and processes utilizing the mixer and, in particular, a process for converting sulfur containing gas into elemental sulfur.
Bond et al., U.S. Pat. No. 4,051,231, discloses a gas mixer and reactor which includes an elongated gas flow chamber with a nozzle arrangement at its inlet. Atmospheres for kilns having controlled amounts of free hydrogen, carbon monoxide or oxygen, for example, are produced by burning controlled ratios of fuel, air, and in some cases an inert gas mixed by the reactor.
Bond et al., U.S. Pat. No. 4,069,020, discloses a process for the production of reducing gases and an apparatus for use therein. A unique gas mixer and reactor is provided which can be used to effect substoichiometric reactions of gaseous reactants to produce a hydrogen-rich gas. The gases which are to be reduced are then admixed with the hydrogen-rich gas, and the gaseous mixture is passed to a catalytic reactor where the reduction reaction takes place.
Bond, et al., U.S. Pat. No. 4,123,220, discloses a gas mixer and reactor which is especially suitable as a burner.
The foregoing references, the disclosures of which are incorporated herein by reference for all purposes, do not, however, address the need for controlling the hydraulic loading or the concentration of hydrogen sulfide in the acid gas feedstock of a Claus type sulfur recovery unit.